The long-term goal of this investigation is to understand the molecular mechanisms underlying tissue-type specific tumor predisposition by the inactivational mutation of the tumor suppressor genes. The model system for study is retinoblastoma, a childhood ocular tumor. The gene involved, RB-1, was isolated by a positional cloning approach in 1987. The retinoblastoma gene product is ubiquitously expressed and has many interesting properties that indicate it may play a regulatory role in the cell division cycle. The observation of the limited spectrum of tumor types in the human retinoblastoma pedigree, on the other hand, is paradoxical to the current hypothetical function of the RB gene. To further address this issue, we have used homologous recombination to inactivate one RB allele in mouse embryonic stem cells and have established a mouse strain carrying a germ-line mutation of the RB gene. Heterozygous mice do not develop retinoblastoma, but are predisposed to brain tumor with greater than 95% penetrance. The following aims are proposed to study the function of the RB gene at the cellular and molecular levels: A) The neuronal tumors in the heterozygous mice will be characterized by immunostaining, the expression pattern of various markers will be compared to that of the human retinoblastoma. These studies will provide the basis of tumor predisposition of specific tissue type in two different species, humans and mice. B) The functional consequences of RB inactivations in the neuronal cells will be studied. Whether deregulated neuronal cell proliferation precedes cell death will be studied. Genes whose expression has been altered following RB inactivation will be characterized. These studies will reveal the molecular events that might be related to the RB function in the nervous system. C) The functional consequences of RB inactivations in the hematopoietic stem cells will be studied by transplantation of the stem cells into the wild-type recipient mice. These approaches will differentiate the cell-autonomous or non- autonomous function of the RB gene in the hematopoietic lineage. D) The developmental and tumorigenic potential of the homozygous RB-deficient embryonic stem cells will be followed in a wild-type mouse background. These studies will provide a global view of the RB function in all cell types. E) A tissue-specific inactivation of the wild-type RB allele in the heterozygous mice will be developed using the yeast FLP recombinase system. These experiments will lead to a defined system where the tissue- specific function of the gene can be dissected at greater detail.